Digging up on the Paizo forums and the SRD, this is what I found:
"If used as part of a full attack action, attacks with a tail blade are considered secondary attacks."
Source:
http://www.d20pfsrd.com/equipment---final/weapons/weapon-descriptions/tailblade
On secondary attacks this is what I found: "Creatures with natural attacks and attacks made with weapons can use both as part of a full attack action (although often a creature must forgo one natural attack for each weapon clutched in that limb, be it a claw, tentacle, or slam). Such creatures attack with their weapons normally but treat all of their available natural attacks as secondary attacks during that attack, regardless of the attack’s original type."
Source:
http://www.d20pfsrd.com/bestiary/rules-for-monsters/universal-monster-rules#TOC-Natural-Attacks
So... lets say your STR modifier is +4 and your BAB is +5 (no other things apply). You are dual wielding kukris and have a tail blade, you have also have the two weapon fighting feat.
Your full attack routine is as follows.
+7 Main hand Kukri (+4STR, +5 BAB, -2 Two weapon penalty) for 1d4+4 DMG.
+7 Off-hand Kukri (+4STR, +5 BAB, -2 Two weapon penalty) for 1d4+2 DMG.
+4 Secondary Natural Attack with the Tail Blade (+4STR, +5 BAB, -5 Secondary Natural Attack penalty) for whatever damage-die-the-tail-blade-deals +2.